The Lesch-Nyhan syndrome is a devastating and ultimately fatal neurological disorder caused by the complete deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT). This disorder has been identified as one of the initial candidates for somatic cell gene replacement therapy. Several other groups are developing retroviral vectors to transfer the human HPRT gene to hematopoietic stem cells. However, several lines of evidence argue against the efficacy of this approach with respect to correction of the enzyme deficiency in the central nervous system. In our currently funded grant, we proposed construction of a neurotropic vector, derived from herpes simplex virus type 1 (HSV-1) to transfer an expressible human HPRT cDNA to neuronal tissue. We have constructed one such virus which expresses human HPRT in cultured rat neuronal cells. In this construct, replication of HPRT cDNA, and hence expression of HPRT activity, requires replication of the viral genome. Thus, the recombinant virus remains virulent, although to a lesser degree than wild-type HSV-1. In this proposal, we describe an alternative approach to the development of an HSV-1 derived vector. The aim is to construct a replication defective virus capable of transferring HPRT cDNA within an autonomously replicating unit. Murine autonomously replicating sequences (ARS) will be linked to an HPRT minigene in which human HPRT cDNA is under the control of the HSV-1 thymidine kinase promoter. This construct will be inserted into an HSV-1 mutant from which the gene encoding ICP4 has been deleted. ICP4 is required for viral replication and this mutant is therefore replication defective. Passage of the recombinant virus through a packaging cell line, which has been transformed by the wild-type ICP4 gene, allows viral replication and assembly of mature virus particles. These virions, however, remain replication defective. HPRT deficient neuronal cells will be infected with these defective recombinants. Although viral replication will not occur, the ARS/HPRT minigenes should replicate as episomes. This innovative approach to the construction of DNA viral vectors circumvents certain problems inherent in replication-competent vector systems. However, several critical experimental questions will need to be addressed during development of such vectors. For these reasons, this proposal meets the standard of "high risk" required by this grant program.